Field of the Invention
The present invention relates generally to atmospheric entry vehicles and deceleration apparatus associated therewith, and more specifically to decelerator devices employed to slow the descent of payloads during atmospheric entry.
Description of the Related Art
A complicated aspect of space travel is the atmospheric entry phase, wherein the space system enters the atmosphere of the Earth or other planets and moons with atmospheres and can be subjected to extreme heat conditions. Certain payloads need to enter the atmosphere and descend safely to the surface, but the challenges involved are many. A number of solutions have been employed, but in general, there is a strong desire to provide a highly efficient and lightweight solution.
Entry vehicles with payloads typically employ heat shielding. Current rigid atmospheric entry heat shields, or aeroshells, are limited by the size of the shroud (launch vehicle payload fairing). By nature, this means that the size must be relatively small even for very large payloads. For a given system mass, the smaller the characteristic size of the aeroshell, the higher the heating rate during entry. Such heat shields must be made of insulating materials that can tolerate very high temperatures and stagnation pressures. Even on the back surfaces, the temperatures of gases flowing around the heat shield are very high, typically requiring a rigid aft heat shield covered with insulating material resistant to high temperatures to protect the payload. The thermal protection system (TPS) materials are typically ablative or high-temperature reflective type materials that reduce heat flux into the payload or vehicle. TPS requires weight, structural support, and volume that displace payload weight and volume.
Past and current research and development of inflatable aerodynamic decelerators (IAD), such as the NASA Inflatable Re-entry Vehicle Experiment (IRVE), and the Low-Density Supersonic Decelerator (LDSD), increase the area of the heat shield as compared with a rigid aeroshell. However, temperatures and stresses are still very high and require special softgoods to withstand the environment and bulky inflation systems.
Current deployable aeroshells are very complex with many technical challenges yet to be overcome. They rely on one or more internal, inflated envelopes or bladders contained within a high-temperature fabric support structure. These designs are sometimes referred to as tension cone or stacked toroidal tension cone systems. Such inflated decelerators require less stowed volume than rigid aeroshells and as a result larger payloads can be launched with smaller launcher shrouds. However, while these inflated aerodynamic decelerators can be somewhat larger in size than a rigid aerodynamic decelerator for the same payload mass and decelerator mass, they are still small enough to result in relatively high heating rates.
Because these types of inflatable aerodynamic decelerators are opaque to entry heat flux and can only radiate from the surface facing the flow, heat can build up on surfaces and create damaging temperatures. For this reason, TPS materials, like ceramic fabrics (e.g. Nextel fabrics), are applied to protect underlying materials and inflated bladders, especially at the stagnation point and around the edges where the radius of curvature is small. Localized heating is significantly higher when the radius of curvature of the surface is smaller. As with the rigid aeroshells, TPS materials add extra mass and volume to the inflatable aerodynamic decelerators reducing mass and volume available to the payload.
All these complexities lead to fabric structures with relatively high areal densities (mass per unit projected area). The successful IRVE-2 system launched in 2009 had a rough projected area (cross-sectional area) areal density of about 2,100 g/m2 (mass of envelope divided by the projected area facing the flow). This mass does not include inflation, control and support structure masses, which for the IRVE system required over 80% of the mass of the entry system (˜70 kg vs. the ˜15 kg envelope). If the mass of the overall system is constrained, as it usually is, this implies reduced payload mass.
Current rigid atmospheric entry aeroshells thus require relatively large mass and volume for TPS and a large launch vehicle shroud for their use, which limits the entry payload capability or requires larger and expensive launch vehicles for their deployment in orbit than otherwise would be needed. Past and current inflatable aerodynamic decelerators under research by NASA and others, which may have attitude controllability, can be stowed in smaller volumes, but still require substantial mass and volume for high-temperature TPS materials thus severely reducing payload mass capability.
Some inflatable aerodynamic decelerators are configured as large drag bodies trailing behind the rigid portions of the system. Such a decelerator may be called a ballute, which is a contraction of balloon and parachute. A ballute can potentially increase the drag area more than an inflatable torus around the periphery of a rigid aeroshell because it is not confined to fit in the annular region around the rigid aerosphell and the launch vehicle payload fairing during launch. Ballutes described in the literature may be attached directly to the aft portion of the rigid part of the system or may trail on cables. In either case, the rigid portion of the system is still exposed to the oncoming hypersonic flow, which requires the use of thermal protection system (TPS) materials to prevent damage due to the very high temperatures of the flow field interacting with the solid surface.
In light of the foregoing, it would be advantageous to offer a light decelerator design that decreases the heat issues and other drawbacks of previous entry system designs.